Electric governor system for sensing and controlling engine speed



M. l. ROSENBERG ET Al. 3,414,771 ELECTRIC GOVERNOR SYSTEM FOR SENSING AND CONTROLLING'ENGINE SPEED 2 Sheets-Sheet l Dec. 3, 1968 Filed Nqv. l2, 1965 Rosi-:NBERG ET Al. 3,414,771 ELECTRIC GOVERNOR SYSTEM FOR SENSING AND Dec. 3,

CONTROLLING ENGINE SPEED 2 Sheets-Sheet 2 Filed Novv. 12, 1965 FIGB.

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United States Patent O 3,414,771 ELECTRIC GOVERNOR SYSTEM FOR SENSING AND CONTROLLING ENGINE SPEED Merton I. Rosenberg and Walter G. Bohaker, Springfield, Mass., assignors to American Bosch Arma Corporation, Garden City, N.Y., a corporation of New York Filed Nov. 12, 1965, Ser. No. 507,392 5 Claims. (Cl. 317-5) ABSTRACT 0F THE DISCLOSURE A pick-up coil responds to rotation of the teeth of a gear in an engine to produce a signal having a frequency proportional to enginev speed. This signal is passed in series through a saturating amplifier, which removes R.M.S. variations, through a high-pass filter having a steep, adjustable low-frequency passband edge, and then in succession through a rectifier, a low-pass filter and a D.C. amplifier to the control winding of a rotary solenoid for operating the fuel-control in a direction to oppose engine speed changes when said frequency increases into the range of said passband edge.

This invention relates to engine systems in which the speed of the engine is automatically controlled or governed, and particularly to electrical governor apparatus for use therein.

It is well known in the art to control automatically the fuel supply for an engine so as to maintain its speed substantially at a predetermined value despite changes in load on the engine or changes in environmental conditions, for example. It is also known to provide such systems in which the speed which is maintained by the governing apparatus can be changed controllably, either for different applications or as a throttle for permitting adjustment of the governed speed during one particular use of the system. In addition to mechanical systems of this nature, various electric governors have been proposed which derive electrical signals indicative of engine speed, process these signals, and use them for controlling the fuel supply to the engine so as to maintain the desired engine speed. The present invention falls in the latter class of electrical system.

While a number of such systems employing electric governors are known, their reliability, simplicity, accuracy of regulation and ease of speed adjustment have been less than are desirable in certain applications.

Accordingly it is an object of the invention to provide a new and useful governed engine system, and new and useful electrical governor apparatus suitable for use therein.

Another object is to provide such a system, and electrical governor apparatus for use therein, which is particularly advantageous in respect of one or more of its reliability, simplicity, accuracy of regulation and ease of speed adjustment,

In accordance with the invention, these and other objects are achieved by the provision of an electric governor apparatus which-employs speed-sensing means for deriving an original electrical signal having a frequency and an amplitude varying with the speed of an engine to be governed, and in which this original electrical signal is supplied to limiting means which convert it to a modified signal which varies in frequency with said engine speed but has an R.M.S. value which is substantially invariant with changes in said engine speed over a substantial range of engine speeds. The resulting modified signal is supplied to frequency-sensing means which produces an output signal varying in R.M.S. value with the frequency of said modified electrical signal over at least a ICC part of the range of frequencies produced by changes in said engine speed; the latter output signal also responds to any changes in R.M.S. value of signals supplied to the frequency-sensing means, but since the above-mentioned limiting means has removed variations in R.M.S. value from the electrical signal the output signal from the frequency-sensing means depends only on the frequency of the input signals thereto and hence only on engine speed. The output signal from the frequency-sensing means is then supplied to means for controlling the speed of said engine to maintain it substantially at a predetermined value.

In accordance with further features of a preferred embodiment of the invention, the speed-sensing means is disposed adjacent the path of travel of the teeth of a loadbearing gear driven by the engine and comprises a device, such as a magnet with a coil about it, which is responsive to passage by it of successive teeth of the gear to produce the original electrical signal. The limiting means is preferably a saturating amplifier supplied with the original electrical signal from the speed-sensing means for producing a pulse signal comprising a train of pulses recurrent at the frequency of the original electrical signal but of substantially constant amplitude; the widths of the latter pulses vary inversely with the frequency of the original signal and hence the pulse train has a substantially constant R.M.S. value despite changes in frequency. The frequency characteristic of the frequency-sensing means preferably has asteep lower-frequency skirt extending over a limited band of frequencies including the frequency of recurrence of said pulses for some operating speeds of the engine, whereby the output of the frequencysensing means comprises a train of pulses recurrent at the frequency of the input pulses thereto but having amplitudes which vary as a monotonie function of their recurrence frequency; preferably the frequency-sensing means comprises a Zobel filter, such as a constant-K or an mderived filter, and includes a variable element such as a variable inductor to permit shifting of the frequency position of the lower-frequency skirt. The range of frequencies over which the lower-frequency skirt can be adjusted is preferably large compared with the band of frequencies occupied by the skirt at any given time.

Preferably the output of the frequency-sensing means is passed through a rectifying and DC filtering circuit and thence through a DC amplifier to an electrical actuator which moves the fuel control for the engine to a position determined by the strength of the signal from the DC amplifier. The arrangement is such that, upon starting, the fuel control is held wide open by 'a large current from the DC amplifier until the frequency of the original signal produced by the speed-sensing means reaches the band of frequencies occupied by the lowerfrequency skirt of the frequency-sensing means, after which the -control signal decreases in proportion to further increases in the speed of the engine; this reduces the fuel supply until the engine speed stabilizes at a value dependent upon the frequency position of the lowerfrequency skirt of the characteristic of the frequencysensing means. Having reached this position the engine speed will be stabilized against factors such as load variations, due to the servo mechanism arrangement just described by which increases in engine speed are caused to decrease fuel supply. By adjusting the position of the lower-frequency skirt of the frequency-sensing means, the regulated speed can be adjusted over a substantial range. Because the system responds only to frequency changes in the original signal, and not to amplitude changes therein, the system operates reliably and accurately despite the occurrence of electrical interference or changes in the sensitivity or effective gain of the portion of the system preceding the frequency-sensing means. Accuracy and reliability are also enhanced by the use of the mderived or constant-K type of filters, which also permit ready 'adjustment of regulated speed. The preferred form of sensing means, which derives its signals from the teeth of a load-bearing gear driven by the engine, greatly simplifies installation of the subject electric governor in existing engines.

Other objects and featu-res of the invention will be more readily understood from a consideration of the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE l is a schematic diagram, partly in block form, illustrating one preferred embodiment of the invention;

FIGURES 2A, 2B, 3A, 3B and 4-9 inclusive are graphical representations to which 4reference will be made in describing the characteristics and operation of the illustrated embodiment of the invention.

Referring now to the embodiment of the invention shown in FIGURE l by way of example only, there is illustrated an engine 10, such as a diesel or gasoline engine for example, having a fuel control 12 operable to various positions to control the fuel supply to engine in response to electrical signals supplied to control winding 14 of electrical actuator 16, each value of electrical current through the control winding 14 producing a corresponding predetermined adjustment of fuel control 12. The actuator is preferably spring-biased in the direction to decrease fuel supply, increasing current through winding 14 lserving to increase the fuel supply.

One mechanical output of engine 10 is coupled to shaft of toothed gear 22, which is shown as meshing with Iand driving another toothed gear 24 the shaft 26 of which is connected to Idrive a load 28. Accordingly, both gears 22 and 24 are load-bearing gears doing useful work. They may be gears used to drive the main load on the engine, or they may operate into some load ancillary to the operation of the engine; for example they may drive the flywheel of the engine or operate a fuel injection pump for the engine.

At least one of the gears, e.g. gear 22, is of a magnetic material and has disposed adjacent the path of the tip of its teeth 4a speed-sensing means or pick-up 30 comprising in this example a permanent magnet 32 having a coil 34 of copper wirewound around it, this pickup being disposed sufficiently close to the path of the tip of the `gear teeth that there is induced in coil 34 an electrical signal of a frequency equal to the frequency of passage by it of the gear teeth. In the present form of device the voltage induced in coil 34 is substantially sinusoidal in form; FIGURE 2A represents the variation in voltage from coil 34 as a function of time for a given rate of rotation of gear 22, and hence for a given speed of engine 10, while FIGURE 2B shows the same voltage for the case in which gear 22, and hence engine 10, are operating at half the speed represented by FIGURE 2A. The frequency of this original elect-rical signal produced in coil 34 varies in direct proportion to the speed of engine 10.

The electrical output of coil 34 is applied to a saturating AC amplifier shown enclosed within the dotted block 36. This amplifier serves as a limiter for removing amplitude variations from the original signal applied to its input from pick-up 30. This characteristic is represented in FIGURE 6| in which ordinates represent R.M.S. volts and `abscissae represent speed of engi-ne 10 in revolutions per minute. Curve A represents the R.M,S. value of the original electrical signal applied to the input terminals of amplifier 36 over leads 40 and 42, while curve B in FIGURE 6` represents the R.M.S. output voltage of amplifier 36 appearing between output leads 44 `and 46. It will be seen that while t-he original signal from pick-11p 30 increases substantially linearly for engine speeds from below about 400' r.p.m. to about 3600 r.p.m., at about 400 r.p.m the amplifier output voltage lreaches a maximum value which it maintains substantially constant up to about 3600 r.p.m.

The saturating amplifier utilized in the preferred embodiment to produce this limiting of the input signal and a substantially constant R.M.S. value of output voltage comprises a pair of cascaded AC-coupled transistor arnplifier stages. More particularly, the leads 40 and 42 from pick-up 30 are connected, respectively, to the base 50 of an NPN transistor 52 and to a common ground connection 53. The emitter 54 of transistor 52 is connected directly to ground, while its collector 56 is connected through a load resistor 58 to a source of positive potential designated B-|-, by way of a manually-operable power switch 60 which can be opened to discontinue operation of the electric governor. The output signal at collector 56 is AC-coupled -by way of capacitor 62 to the base 64 of another NPN transistor 66. Preferably both transistors 52 and y66 are of the silicon type. The emitter 68 of transistor 66 is connected directly to ground, While its collector 70 is connected by way of load resistor 72 to the positive supply voltage B+, again by Way of the power switch 60. Bias for the base 64 of transistor 66 is provided by means of a voltage divider arrangement comprising resistor 74, connected between collector and base of transistor l66, and resistor 76 connected between base 64 and ground.

In one representative embodiment transistors 52 and 66 may be silicon type 2N2926 transistors, resistors 53, 72, 74 and 76 may have respective values of 470 ohms, 250 ohms, 5,000 ohms and 1,000 ohms; and capacitor 62 may have a value of eight microfarads.

The effect of the first transistor 52 is to amplify the original input signal to a magnitude sufficient to drive the second transistor 66 strongly between cutoff and collector saturation, so that one half-cycle of the input sinusoidal wave is effectively clipped off entirely while the other half-cycle is converted to a series of substantially rectangular pulses of constant amplitude each having a duration substantially equal to one-half the period of the input sinusoid. Although at higher frequencies there are a greater number of pulses per second at the output of amplifier 36, the widths of the individual pulses vary inversely with the frequency so that the R.M.S. value remains the same so long as the peak value of the pulses is constant. This is illustrated by FIGURES 3A and 3B, in which ordinates represent the amplitude of signal at the output of saturating AC amplifier 36 and abscissae represent time, the pulses of FIGURE 3A being produced in response to the sinusoid of FIGURE 2A and those of FIGURE 3B being produced by the sinusoid of FIG- URE 2B. By comparing FIGURE 3A with FIGURE 3B it can be seen that, While at the frequency corresponding to FIGURE 2A four rectangular pulses are produced in the interval occupied by two pulses in FIGURE 3B produced by the sinusoid FIGURE 2B, the pulses of FIG- URE 3B are twice as long in duration as those of FIG- URE 3A and hence the total energy of the train of pulses, and thus their R.M.S. value, in a given time is the same in both cases, i.e. regardless of frequency of sinusoidal input.

The output of saturating AC amplifier 36 comprising a series of rectangular pulses Of constant amplitude recurrent at the frequency of the original sinusoidal input signal is applied to the input of AC filter and speed control circuit 80, by way of the large AC coupling capacitor 82. The AC filter comprises a frequency sensing circuit which produces output pulses having amplitudes dependent upon their frequency of recurrence. Preferably `the AC filter is a filter of the so-called Zobel class, comprising constant-K or m-derived filter arrangements, and is of the high-pass type with a frequency characteristic having a steep lower-frequency skirt extending through a band including frequencies equal to the repetition rate of pulses applied thereto from amplifier 36. The frequency position of this lower-frequency skirt is preferably variable manually or mechanically.

More particularly, in the present embodiment AC filter and speed control circuit 80 comprises a symmetrical arrangement of equal-valued capacitors 84 and 86 connected together at a junction point 88, the other 'terminal of capacitor 84 being connected to coupling capacitor 82 and the other terminal of capacitor 86 being connected to output lead 90 of the AC filter and speed control circuit. A pair of symmetrically-disposed equal-valued resistors 92 and 94 are connected between ground and said other terminals of capacitors 84 and 86, respectively. The junction point 88 is connected to ground through a variable inductor 98 in series with a capacitor 100, and another pair of resistors 102 and 104 are bridged across capacitors 84 and 86, respectively.

In one representative embodiment, capacitors 84 and 86 may have values of 0.12 microfarad, resistors 92 and 94 may have values of 1,000 ohms, resistor 102 may have a value of 400 ohms and resistor 104 a value of 750 ohms, while capacitor 100 mayhave a value of 0.5 microfarad. Inductor 98 may have a variable value in the general region of one henry.

FIGURE 4 illustrates the general principle of operation of the AC filter. The curve is a plot with frequency as abscissae, and relative response of the filter as ordinates. It will be seen that the filter characteristic is of the high-pass type with a steeply-rising lower-frequency skirt C. Ordinate line A in FIGURE 4 represents the amplitude of pulses out of the AC filter in response to the relatively rapidly recurrent input pulses; ordinate line B represents the output of the AC filter in response to inp-ut pulses recurrent at a lower frequency. It can be seen that as the recurrence frequency of the input pulses to AC filter and speed control circuit 80 increases through the lfrequency band D occupied by the lower-frequency skirt C of the filter characteristic, the amplitude of the output pulses produced thereby from the filter increases substantially monotonically, and substantially linearly.

By varying the inductance of inductor 98 the frequency position of the skirt C of the frequency characteristic of FIGURE 4 can -be varied at will. This is represented in FIGURE 7, in which ordinates represent R.M.S. volts from the AC filter and speed control circuit 80, and ordinates represent engine speed in r.p.m. The curves a, b, c, d and e show the frequency positions of the lowerfrequency skirt of the frequency characteristic of the AC filter for different adjustments of inductor 98. From FIGURE 7 it can be seen that this frequency position can be varied over a wide range, for example over the entire range of frequencies produced by engine speeds between 800 and 3600 r.p.m., and that this range of variation of the position of the skirt is many times greater than the frequency 4band occupied by the skirt in any given adjustment. As will be explained later, the steep slope of the skirt assures accurate and reliable regulation, while the controllable variation of the frequency position of the skirt permits adjustment of the regulated speed while retaining such accuracy and reliability of regulation.

The pulses from AC filter and speed control 80 having an amplitude modified in accordance with the relation of their recurrence frequency to the lower-frequency skirt of the frequency characteristic of the AC filter, are then supplied by way of coupling capacitor 105 to a rectifier and DC filter 106 of FIGURE 1. The latter circuit functions to produce at its output leads 108 and 110 a continuous direct voltage which varies in accordance with the amplitude of the train of pulses supplied -thereto from AC filter and speed control circuit 80. This is represented in FIGURE 5, where ordinates represent DC voltage between the output leads 108 and 110 of rectifier and DC filter circuit 106. Line A in FIGURE 5 represents the direct voltage there produced in response to the higher-frequency pulses corresponding to ordinate A in FIGURE 4, while line B represents the direct voltage produced in response to lower-frequency pulses corresponding to ordinate B in FIGURE 4. Accordingly there is produced between the output leads 108 and 110 of rectifier and DC filter circuit 106 a slowly-varying direct voltage having a magnitude substantially proportional to the pulse recurrence frequency when the latter frequency lies within the band D occupied by the skirt C of the frequency characteristic of AC filter and speed control circuit 80.

Rectifier and DC filter circuit 106 may take any of a large variety of known forms. In the particular embodiment illustrated in FIGURE 1, it comprises a voltage doubler `arrangement consisting of a diode 114 having its anode grounded and its cathode connected to a junction point 115 between coupling capacitor 105 and another diode 116. The anode of diode 116 is connected to junction point 115 and the cathode of the latter diode is connected to a filtering and smoothing circuit consisting of a series inductor 118 and a pair of filter capacitors 120 and 122 connected respectively between ground and opposite ends of inductor 118. The terminal of inductor 118 opposite to that connected to diode 116 is connected to output lead 108. This circuit arrangement is effective to provide the above-described operation for Producing between output leads 108 and 110 a direct voltage varying substantially only with engine speed.

In one representative embodiment, inductor 118 may have an inductance of about 4.5 henries and a resistance of 300 ohms, and capacitors 120 and 122 may each have a capacity of 34 .microfarads The output of rectifier and DC filter circuit 106 is supplied to the input of a DC amplifier which serves to amplify the DC signal and apply it in the proper polarity to control winding 14 of electrical actuator 16. Any of a variety of known types of DC amplifiers may be utilized for this purpose. The particular DC amplifier shown in FIGURE 1 utilizes three transistor stages in cascade.

More particularly, in the representative embodiment illustrated in FIGURE 1 DC amplifier 130 is constructed as follows. A voltage divider comprising three seriesconnected resistors 132, 134 and 136 is connected between the output lines 108 and 110 of rectier and DC filter circuit 106. Resistors 132 and 136 have variable tap connections 138 `and 140, respectively. Variable tap 138 is connected to the base of an NPN transistor 142, the emitter of which is connected to the base of a second NPN transistor 143. The collector of transistor 142 is connected by way of collector load resistor 146 and regulating resistor 148 to power switch 60 and thence to the source of positive potential designated B+; the collector load resistor 150 of transistor 143 is similarly connected by way of resistor 148 to the positive supply. The emitter of transistor 143 is connected directly to ground.

The collector of transistor 143 is also connected by way of a smoothing inductor to the base of a third transistor 162, the emitter of the latter transistor being connected to ground by way of a resistor 164. The collector of transistor 162 is connected to power switch 60 and the source of positive potential by way of the control coil 14 of electrical actuator 16. An AC negative feedback path is provided from the base of transistor 162 to the base of transistor 142 by way of a series capacitor and a variable series resistor 172 for stabilizing purposes, adjustment of resistor 172 permitting adjustment of the extent of the negative feedback. A DC negative feedback path is provided from the emitter of transistor 162 to the variable tap 140 on resistor 136, and hence also to the base of transistor 142, by way of resistor 176, to serve as an anti-hunt circuit. A manually operable switch 178 is connected across resistor 176 so that, when the engine is operating at low speeds, the feedback resistor 176 can be short-circuited to provide increased negative feedback and decreased gain. A voltage-regulating Zener -level determined by the adjustment of inductor 98 an increasing positive voltage is applied to the base of tran- .sistor 142. This causes transistor 142 to become more conductive and makes the base of transistor 143 more positive. The latter transistor therefore becomes more conductive and its collector voltage more negative, reducing conduction in transistor 162. This corresponds to a reduction in the current through control winding 14 of electrical actuator 16.

Electrical Iactuator 16 typically comprises an electrical winding and armature arrangement, with the armature connected by a suitable linkage to fuel control 12 and arranged so that increasing current through control winding 14 moves fuel control 12 in the direction to supply more fuel -to engine and hence to speed up the engine. Accordingly, the increasing positive voltage applied to the input to the DC amplifier 130 when engine 10 iS speeded up sufficiently to produce pulse recurrence frequencies in the frequency band occupied by the skirt of the AC filter, decreases the current `through actuator control winding 14 and causes fuel control 12 to reduce the supply of fuel so as to slow down engine 10. This action continues until a balance is reached and the engine speed stabilized at a point determined by the setting of inductor 98.

The latter action is illustrated in FIGURE 7; if it is assumed that inductor 98 is set so that the frequency characteristic of AC filter and speed cont-rol circuit 80 is that shown in curve a, the engine speed will be stabilized at a speed such as that indicated by the dotted line a' and at a voltage from the AC filter corresponding to the intersection of the dotted line a and the characteristic curve a. Should the engine speed tend to depart from this value, due to load changes for example, it will cause electrical actuator 16 to move the fuel control 12 slightly in the direction to oppose the speed change, the gain of the complete system being sufficient that only small speed changes can occur, corresponding to a high degree of regulation. If inductor 98 is readjusted to move its lowerrequency skirt to the higher frequency position corresponding to curve b in FIGURE 7, the engine speed will be regulated at the higher value corresponding to dotted line b. In this way the regulated speed of'the engine may be adjusted over a wide range, e.g. from below 800 to above 3600 rpm., and held substantially constant at the regulated speed Actuator 16 is preferably of the type described and claimed in the copending application Ser. No. 570,802 of Merton I. Rosenberg, entitled Rotary Electromagnetic Actuator and tiled Aug. 8, 1966, which utilizes a rotary armature spring-biased against the magnetizing force produced by the magnetizing winding 14, and in the present application the spring bias is in the direction to slow down engine 10.

FIGURE 8 is a plot in which ordinates represent current supplied to the input of the electrical actuator and abscissae represent the speed of engine 10 in r.p.m. The several curves a, b, c, a and e represent the characteristic obtained, respectively, for the different frequency characteristics a, b, c, d and e of FIGURE 7, produced by different adjustments of inductor 98, when the servo loop is opened, as by opening the linkage between electrical actulator 16 and fuel control 12. These characteristics of FIG- URE 8 indicate that the actuator current is at a fixed high level until the engine speed has risen to the point where the pulses produced thereby are recurrent at a frequency in the baud occupied by the particular lower-frequency skirt of the AC filter then in effect, at which time the CFI actuator current decreases sharply with increasing engine speed. With the servo system closed as shown in FIGURE l, the engine speed is regulated and locked at a particular predetermined value, such as a in FIGURE 8 for the case Where the characteristic curve a is in effect.

FIGURE 9 is a plot in which ordinates represent the load on engine 10 and abscissae represent the speed of engine 10 in r.p.m., the five curves a, b, c, d and e again corresponding to the above-described five different adjustments of the AC filter. The latter curves are not necessarily to scale, but merely indicate the nature of the action obtained. Thus if the horizontal line F.L. represents full load on the engine, it can be seen that with the regulating and governing system operating the engine speed can be set at any desired value from below 800 to above 3600 r.p.m. and will not change substantially despite variations from zero to full load on the engine.

It will therefore be appreciated that an electrically governed engine system has been provided which can readily be installed in existing types of engine systems since it can derive its input signal directly from existing load-'bearing gears driven by the engine, and can apply its output by way of existing types of linkages operating the fuel control. The system is accurate and reliable because it responds only to changes in the frequency of the electrical pick-up signals and not to their strengths, once the engine speed has risen into the speed range for which governing is to be provided, and because the type of AC filter utilized provides a rapid change of voltage with pulse frequency within the band of frequencies for which control is exerted. It also provides for simple and accurate adjustment of the governed speed over a wide range while preserving the above-mentioned desirable characteristics of the system for all adjustments within the range.

While the invention has been described with particular 'be embodied in a wide variety of forms without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. Electrical governor apparatus, comprising:

speed-sensing means responsive to rotation of an engine-driven member to produce an original electrical signal varying in frequency in proportion to the speed of said engine and increasing in amplitude with increases in said speed;

amplitude-limiting means, and means for applying said original electrical signal to said amplitude-limiting means, for producing a pulse signal comprising a train of pulses recurrent at the frequencies of said original electrical signal and of substantially constant R.M.S. value;

first filter means having a frequency passband characteristic with a steep lower-frequency skirt extending over a limited band of said frequencies of recurrence of said pulses, and means for applying said pulse signal to said first filter means, to produce a modified pulse signal comprising a train of pulses recurrent at said recurrence frequencies in said band but of amplitudes which vary `as a monotonie function of said recurrence frequency within said band;

a low-pass filter, and means for applying said train of modified pulse signals to said low-pass filter, for producing a continuous electrical signal of a magnitude varying with the R.M.S. value of said train of pulses from said first filter means;

an actuator control circuit and means for applying said continuous electrical signal to said control circuit to develop a control signal; and

an electrical actuator land means for applying said control signal to said actuator for controlling said speed of said engine in dependence upon the frequency relation between said recurrence frequency of said pulses supplied to said first filter means and the frequency band corresponding to said skirt of said frequency characteristic of said first filter means.

2. Electric governor apparatus, comprising:

speed-sensing means responsive to rotation of an engine-driven member to produce an original electrical signal varying in frequency in proportion to the speed of said engine and increasing in amplitude with increases in said speed;

saturating amplifier means and means for applying said original electrical signal to said amplifier means for producing a pulse signal comprising a train of pulses recurrent at the frequencies of said original electrical signal but of substantially constant amplitude;

first filter means having a frequency passband characteristic with a steep lower-frequency skirt extending over a limited band of said frequencies of recurrence of said pulses, and means for applying said pulse signal to said first filter means, to produce a modified pulse signal comprising ya train of pulses recurrent at said recur-rence frequencies in said band but of amplitudes which vary as a monotonie function of said recurrence frequency within said band;

a low-pass filter and means for applying said train of modified pulse signals to said low-pass filter for producing a continuous electrical signal of a magnitude varying with the R.M.S. value of said train of pulses from said first filter means; and

an electric actuator and means 4for applying said con tinuous electrical signal to said actuator for controlling said speed of said engine in dependence upon the frequency relation between said recurrence frequency of said pulses supplied to said first filter means and the frequency band corresponding to said skirt of said frequency characteristic of said first filter means.

3. The apparatus of claim 2, in which said frequency band occupied by said skirt is small compared with the range of frequencies of said original signal producible by operation of said engine, and in which said first filter Vmeans comprises means for varying said skirt through a range of frequencies large compared with said frequency band occupied by said skirt.

4. The apparatus of claim 2, in which said first filter means comprises a filter of the class comprising constant-K and m-derived filters.

5. The apparatus of claim 4, in which said filter means comprises a controlledly variable reactor for shifting the frequency position of said band occupied by said skirt.

References Cited UNITED STATES PATENTS 2,762,464 9/1956 Wilcox 317-5 X 2,941,120 6/1960 Harman et al 317--5 3,332,406 7/1967 Perry et al. 317-5 X JOHN F. COUCH, Primary Examiner.

l. A. SILVERMAN, Assistant Examiner. 

